The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The near infrared wavelengths extend from 0.75 micrometers to 10 micrometers. The far infrared wavelengths cover the range from approximately 10 micrometers to 1 millimeter. The function of infrared detectors is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects will dissipate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared frequency energy in one of several ways. Thermal detectors respond to infrared frequency energy by absorbing energy, thus causing an increase in the temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistivity, to change. By measuring this change the infrared radiation is measured.
Photo-type detectors (e.g., photoconductive and photovoltaic detectors) absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which, in turn, leads to either a change in the electrical conductivity (photodetectors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is effected is a function of various factors including the particular detector material selected, the doping density of that material and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of focusing lenses constructed of materials transparent to infrared frequency energy, advances in semiconductor materials and highly sensitive electronic circuitry have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connected to sophisticated processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitation, the processing circuitry can identify and monitor sources of infrared radiation. Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and economical production of the detector array and accompanying circuitry.
A contemporary subarray of detectors may, for example, contain 256 detectors on a side, or a total of 65,536 detectors, the size of each square detector being approximately 0.0035 inches on a side with 0.0005 inches spacing between detectors. The total width of such a subarray would therefore be 1.024 inches on a side. Thus, interconnection of such a subarray to processing circuitry requires a connective module with sufficient circuitry to connect each of the 65,536 detectors to processing circuitry within a square a little more than one inch on a side. The subarrays may, in turn, be joined to form an array that includes 25 million detectors or more. Considerable difficulties are presented in aligning the detector elements with conductors on the connecting module and in isolating adjacent conductors in such a dense environment.
The outputs of the detectors must undergo a series of processing steps in order to permit derivation of the desired information. The more fundamental processing steps include preamplification, tuned bandpass filtering, clutter and background rejection, multiplexing and fixed noise pattern suppression. By providing a detector connecting module that performs at least a portion of the signal processing functions within the module, i.e. on integrated circuit chips disposed adjacent the detector focal plane, the signal from each detector need be transmitted only a short distance before processing. As a consequence of such on-focal plane or "up front" signal processing, reductions in size, power, and cost of the main processor may be achieved. Moreover, up front signal processing helps alleviate performance, reliability, and economic problems associated with the construction of millions of closely spaced conductors connecting each detector element to the main signal processing network.
In order to exploit the advantages of on-focal plane processing of the input signals, it is desirable to provide a detector module that can provide the necessary interconnections between various module processors as well as between module processors and external electronics, with a minimum of wiring congestion and using a minimum of module space, thereby reserving additional module space for on-focal plane processing. The present invention provides an improved method and apparatus to achieve such a result.
In the prior art, a number of detector array modules have been proposed for coupling an array of closely spaced detectors to processing circuitry. Such modules are typically formed such that all connections to and from the module are disposed on a first horizontal layer, with electronic devices and connecting circuitry disposed within the module on one of several stacked horizontal layers interconnected by vertical conductors, known as vias, extending through the layers. A principal shortcoming of those constructions is that a single layer is unduly congested with connections to all detectors and external electronics and must also support a large number of vias extending to the parallel layers. The number of detector elements that may be connected as well as the number of connectors to external electronics that support on-focal plane processors are thereby limited by the size of the connecting layer and the minimum width and spacing of the conductors. Moreover, where the connecting layer becomes overly congested with conductors the electronic devices must be formed on a different layer, requiring conductive vias which further congest the module layers, adding weight and cost to the module and introducing potential performance problems.
Other modules proposed in the prior art address the congestion and space optimization problem by orienting the detector array perpendicular to the plane of the module layers, adjacent to one edge of the module. Such constructions, commonly referred to as "Z-technology architectures", are disclosed in U.S. Pat. No. 3,970,990 to Carson, assigned to the common assignee, and U.S. Pat. No. 4,304,624 to Carson, et al. Z-technology modules are typically formed by stacking multiple layers of thin film substrates and bump bonding an end of each layer to an adjacent row of the detector array. Conductors extending along the surface of the substrates have end portions that are carefully aligned to contact leads from the individual detector elements. Such constructions advantageously avoid wiring congestion associated with connecting all detectors to a single module layer and reduce the accompanying need for vertical vias that detract from the usable space with the module.
One such high density multi-layer integrated circuit module is described in U.S. Pat. No. 4,659,931 issued to Schmitz, et al., the entire disclosure of which is hereby incorporated by reference. The Schmitz module is used for integrating a plurality of detector arrays disposed in the direction normal to the plane of the micro circuits, i.e. the Z-direction. The detector arrays have pixels which are typically less than 100 microns apart and are integrated to the modules by flip-chip bump bonding. To maintain an interconnect directly behind each pixel in the Z-direction, the module layers must be kept very thin, usually less than the pixel center-to-center spacing. The layers, which are ceramic substrates having thin film micro circuits printed thereupon, are laminated using extremely thin thermosetting adhesive sheets.
A principal shortcoming to this laminated multi-layer assembly is the poor dimensional stability that exists due to a mismatch of the coefficients of expansion between the ceramic layers and the thermoset adhesive sheets. Typically, the thermosetting adhesives have a coefficient of expansion which is five to ten times that of the alumina ceramic layers. Furthermore, the infrared detectors often operate at cryogenic temperatures, thus subjecting the module to large range of thermal cycling and stress. Such wide ranges in the temperature environment can thus cause degradation of the detector to module electrical interconnect, thus leading to a gradual loss of operational performance. When very thin detectors are used, the thermal stress is transmitted from the module to the detectors and can cause the fragile infrared detector arrays to crack.
To alleviate this problem, the prior art has provided a thermal-stress barrier, commonly known as a buffer board, which must be used to absorb the thermal stress from the module. The buffer board is placed between the infrared detector arrays and the module and thus provides for electrical and mechanical interconnection therebetween. Thus, the use of a buffer board adds to the component and processing costs, reduces the interconnect yield, and complicates the array alignment process during integration.
The coefficient of expansion mismatch between the substrate layers and the thermosetting adhesive, as well as the problem of non-uniformity in the thickness of the laminating adhesives, limits the number of layers that can be assembled in a single module stack. The more layers provided in a single module stack, the greater the absolute error in the stacking height which must thus be accounted for. Consequently, larger tolerances must be provided for in the buffer board electrical interconnection. Thus, the large dimensional tolerances required in the laminated modules has prevented the precision stacking of a large number of layers to form higher level focal planes.
Other inherent deficiencies found in the prior art adhesively laminated module assemblies include poor mechanical strength, lack of stiffness, outgassing, moisture absorbance, and poor resistance to chemicals and solvents.
Thus, it would be desirable to provide a high density multi-layer integrated circuit module for integrating infrared detector arrays to signal processing circuits wherein the coefficient of expansion of the module is substantially uniform throughout and approximately equal to that of the infrared detector arrays such that a buffer board would not be required. Additionally, it would be desirable to eliminate the non-uniformities in the thickness of the laminating adhesives to reduce the dimensional tolerances required in the fabrication of the infrared detector array interconnects. Furthermore, it would be desirable to provide a module having improved mechanical strength; greater stiffness; less outgassing; and better moisture, chemical and solvent resistance.